When the brake is applied during operation of an elevator with the induction motor (IM) driven by an inverter, the rotating speed of the motor is higher than the frequency of the inverter, and regenerated power is formed in the motor. As this regenerated power flows into the DC circuit of the inverter, a resistor in the DC circuit absorbs the regenerated power.
FIG. 3 shows the configuration of conventional regenerated power consumption. The principal circuit has converter (1) and inverter circuit (2). The three-phase AC is converted to DC by converter (1) and then converted to three-phase AC by inverter circuit (2). The speed of the IM (4) is controlled by the base driver (3).
A regenerated power consumption circuit composed of resistor (5) and switch (6) formed of semiconductor elements is inserted in parallel to the principal circuit. The DC voltage on the two ends of principal circuit capacitor (7) is detected by a voltage detector (8). The signal from said voltage detector (8) is input to a hysteresis comparator (9). The magnitude of the DC voltage is the basis for ON/OFF control of a base driver (10) of switch (6). Thus, the regenerated power formed in the deceleration of the IM (4) can be consumed by resistor (5).
FIG. 4 shows the DC voltage waveform of the principal circuit in the aforementioned operation. As can be seen with respect to the DC voltage of the principal circuit, switch ON level and switch OFF level of said regenerated power consumption switch (6) are set by the comparator (9). As the DC voltage rises with the regenerated power, the circuit is turned ON, the regenerated power is thus consumed; then as the DC voltage falls, the circuit is turned OFF.
When used for an elevator, as shown in FIG. 5, the elevator has an IM (4) as the power source, a cage (12), and a balance weight (13) loaded on winding drum (11). The velocity pattern for acceleration, deceleration, and constant velocity is generated by a control unit (14) so that the cage (12) may be stopped on any floor.
The maximum load depends on the number of passengers in the cage, etc. Because the number of passengers can change, there can be a large regenerated power in the case of deceleration.
In addition, the deceleration rate depends on the velocity pattern and the regenerated power varies, depending on the number of passengers.
Even in the case of constant velocity operation, when the cage (12) is heavier than the balance weight (13) due to more passengers, regenerated power is formed in the descending stage. On the other hand, when the cage (12) is lighter than the balance weight (13), the regenerated power takes place in the rising stage. These regenerated power levels also vary as the number of passengers changes.
As explained above, the regenerated power of the elevator depends on the number of passengers and the operation status--whether deceleration or constant velocity. Consequently, for hysteresis comparator (9), the switching frequency and the ON/OFF ratio also depend on the change in the regenerated power. This dependency makes the design of the regeneration circuit complicated. To realize reliable operation for switch (6), a switch which allows high-speed switching operation up to several kHz must be used. In addition, it is difficult to design the hysteresis width and ON/OFF operation level of the hysteresis comparator (9) and to set the resistance value of resistor (5).